This invention relates to semiconductor fabrication and, in particular, to apparatus for plasma etching coatings on a semiconductor substrate.
During approximately the past decade, plasma etching has received continuously increasing attention as an alternative to wet chemical etching, due to both superior process control as well as environmental considerations. As known by those of skill in the art, plasma etching is largely a chemical process in which an essentially neutral cloud of gas is ionized by RF (radio frequency) energy. The radicals produced in the plasma are chosen to react with the particular material to be etched. For example, carbon tetrafluoride (CF.sub.4) reacts with silicon to produce silicon tetrafluoride (SiF.sub.4), which is a gas at room temperature and can thus be removed by exhausting the chamber containing the silicon wafers.
The plasma etching processes to date have been controlled by a timer. The optimum time is previously determined empirically and depends, as known in the art, on temperature, gas pressure, flow rate, and RF power. It is thus apparent that, even with time preset, the process requires careful control.
It has also been found in the prior art that the luminosity of the plasma changes when the etch cycle is complete. This, however, has only been used as a supplementary system check to assure that the etching has been permitted to continue for an adequate length of time, for "ashing" or complete removal of the photoresist.
In copending patent application Ser. No. 939,586, originally filed June 17, 1976 and assigned to the assignee of the present invention, there is disclosed a plasma developable photoresist exhibiting different etch rates for the exposed and unexposed portions of the photoresist. As with other plasma etch systems, time and other process parameters must be accurately controlled to fully develop the photoresist without removing it entirely. While the different etch rates are beneficial and permit reproducible results, greater accuracy or flexibility, or both, in the process is always desired.